Thermoelectric generator



Nov. 22, 1960 R. w. FRxTTs THERMoELEcTRIc GENERATOR Filed Feb. 21, 1957ss'v" INVENTOR.

Roberf 14/. F r/'fs THERMOELECTRIC GENERATOR Robert W. Fritts, ElrnGrove, Wis., assignor, by mesne asszgnments, to Minnesota Mining andManufacturing Company, St. Paul, Minn., a corporation of Delaware FiledFeb. 21, 1957, Ser. No. 641,540

Claims. (Cl. 136-4) This invention relates to improvements inthermoelectric generators. Thermoelectric generators have long beenutilized as the source of electrical energy for selfpowered controlsystems, one such use finding widespread application being in Safetyshut-off systems for control of the flow of gaseous fuel to the main andpilot burners of gaseous fuel burning apparatus. In systems of this typethe thermoelectric generator is heated by the pilot burner fiame, andthe electrical energy generated is utilized to energize and hold openone or more manually resettable safety shut-off valves. On outage of thepilot burner, the therm-oelectric generator gradually cools, and theoutput thereof diminishes, eventually reaching a reduced levelinsutficient to hold the safety shut-off valve open, whereupon saidvalve drops-out to clo ed position and remains in said position untilsubsequently reset.

The time interval between extinction of the pilot burner fiame andclosure of the Safety shut-olf valve is termed the drop-out time of thesystem. The usual drop-out time of conventional systems found indomestic applications is on the order of 40 to 45 seconds. While thisdrop-out interval is considered safe in domestic systems in which therate of fuel flow is moderate, such a time interval would be extremelydangerous in, for example, industrial applications wherein large volumesof gaseous fuel are consumed per unit time. Substantially reduceddrop-out time is required, therefore, in high capacity systems, andattainment of satisfactory drop-out charaoteristics for suchapplications has heretofore been possible only by resort to complicatedand expensive equipment.

A general object of the present invention is to provide and improvedthermoelectric 'generator the operation of which is characterized by arapid rate of decay in its output voltage following termination of theapplication of heat thereto.

Another object of the invenion is to provide an improvedthermoelectrical generator of the class described 'which for the firsttime makes safe the use of inexpensive Vself-powered Safety shut-offsystems for the control of relatively large capacity systems heretoforerequiring expensive control apparatus.

A further object of the invention is to provide an improvedthermoelectric generator of the aforementioned character which isinterchangeable with the thermocouple of standard construction embodiedin domestic self-powered fuel control systems, said improved generatoraffording such a system substantially reduced drop-out timecharacteristics. Use of the improved thermoelectric generator hasparticular appeal in connection with, for example, Safety shut |off offuel flow to the oven burner of a residential gas range, the reductionin the amount of'escaping gas during the drop-out time not onlyincreasingv the Safety of the system, but also reducing the physicaldiscomfort resulting from breathing of the escaping gas.

WA more specific object ofthe invention is to provide an improvedthermoelectric generator of the class described utilizing athermoelement of semi-metallic material having high thermoelectricpower, said generator being constructed in a manner to afford maximumlife expectancy.

Still another object of the invention is to provide an improvedthermoelectric generator as aforedescribed which is relatively simpleand inexpensive in construction and is well-adapted to mass productionfabrication.

The invention is capable of receiving a variety of mechanicalexpressions, one of which is shown on the accompanying drawing, but itis to be expressly under- Stood that the drawing is for the purposes ofillustration only, and is not to be construed as a definition of thelimits or sc-ope of the invention, reference being had to the appendedclairns for that purpose. In the drawing, wherein like characters ofreference are employed to designate the same parts in each of thefigures:

Figure 1 is a fragmentary elevational view, partly in section, of aSelf-powered Safety shut-off gaseous fuel control system having theimproved thermoelectric generator embodied therein as the pilot burnerfiame sensitive source of power; and

Figure 2 is an enlarged axial sectional view of one form ofthermoelectric generator constructed in accordance with the principlesof the present invention, the lead for said generator being shownfragmentarily and partly in section.

The Safety shut-off fuel control system illustrated in Figure 1 of thedrawing controls the flow of gaseous fuel to a main burner 5 and acoacting pilot burner 6. The main burner 5 may be of a size to consume arelatively large volume of gaseous fuel per unit time, and the fuel flowthereto is under the control of an electromagnetic manually resettableSafety shut-off type valve 7 interposed in the fuel supply pipe 8leading from a source (not shown) of gaseous fuel under pressure. Fuelflow to the pilot burner 6 is under the c-ontrol of an electromagneticmanually resettable safety shut-off type valve 9 interposed in the fuelsupply line 10 connecting the pilot burner to a suitable source (notshown) of fuel under pressure, for example, through a pressure regulatorshown diagrammatically at 11.

Supported, for example, on the main burner 5, isa mounting bracket 12affording mounting means for the pilot burner 6 as well as for athermoelectric generator 4 of novel construction to be described indetail hereinafter. The generator 4 is provided with a coaxial type lead13 having a connector nut 14 and terminal 15.

The Safety shut-off valve 9 comprises a mounting nut 16 for anelectromagnet and valve assembly comprising an electromagnet 17, acoacting armature 18, and a valve member 19 biased toward an annularvalve seat 20 by a spring 21. A Stem 22 connects the valve member 19with the armature 18, so that the bias of the spring 21 also biases thearmature 18 toward its retracted position shown. Manual reset means 23is provided to reset the valve member 19 to open position and tosimultaneously reset the armature 18 into engagement with the pole facesof electromagnet 17. The mounting nut 16 is forrned with a pair ofinternally threaded recesses 24 and 25 having terminal tips 26 and 27insulatably mounted therein respectively. One end of the energzing coilfor the electromagnet 17 is connected in cirzcuit with the terminal tip26, and other end of said coil s grounded to the mounting nut 16 asshown.v The .terminal tips 26 and 27 are connected in circuit with veachother by means of a conductor 28.

The socket 24 of the mounting nut 16 is adapted to receive the connectornut 20 of a coaxial type lead 30, the opposite end of which is providedwith connector means31 for threadedengagement Withpooperating connectormeans 32 on the valve 7. The valve 7 is provided with an electromagnet33 and a cooperating armature 34 which is connected by means of a stem35 to a valve member 3'6. The valve member 36 is biased toward anannular valve seat 37 by means of a spring 38, which, through the stem35, also biases the armature 34 toward retracted position with respectto the electromagnet 33. Manual reset means 39 is provided for resettingthe valve member 36 to open position, and simultaneously resetting thearmature 34 to attracted position with respect to the electromagnet 33.The connector means 31 and 32 afiord electrical connection of theinternal conductor of the lead 30 with one end of the energizing wndingof the electromagnet 33 and of the outer conductor of said lead with thegrounded other end of said winding. It will be observed that in theelectrical circuit so far described, the energizing windings of theelectromagnets 17 and 33 are connected in parallel circuit relation withthe thermoelectric generator 4.

Where it is desired to utilize a single fuel control valve forcontrolling both main and pilot burner fuel, the valve 9 may be omitted,in which case the fuel supply conduit 16 for the pilot burner isconnected to a pilot fuel tap 40 interposed between the valve seat 37and a spaced annular seat 41, and the generator lead is connecteddirectly to the valve 7. Where such an arrangement is used, fiowinterruption is afforded by the use of a fiow interrupter disc 42cooperable with the valve seat 41 and forming part of the reset means39, said disc being operable to interrupt fuel fiow to the main burnerduring resetting of the valve 7, while permitting fuel fiow to the pilotburner 6 during such resetting, all as is well known in the art. Thevalve 7, particularly in large capacity systems, may also take the formof the valve shown and described in the copending application for patentof Donley S. Collins, Serial No. 639,908, filed February 13, 1957, andassigned to the assignee of the present application.

Referring more particularly to Figure 2 of the drawing, thethermoelectric generator 4 takes the form of a thermocouple comprising afirst thermoelement 43 in the form of a tubular cup-shaped sheath memberof stainless steel having a reduced diameter outer end portion 44providing an internal annular shoulder 45. The sheath member 43 iscoaxially fixed, as by silver soldering or brazing at 46, within an endportion of a tubular sheath member or extension sleeve 47, the latterbeing formed with suitable external shoulder means 48 for coaction withthe mounting bracket 12 and mounting nut 49 shown in Figure 1. Fixed, asby silver soldering or brazing at 50, is a tubular sheath member orbushing 51 disposed coaxially within the opposite end of the sleeve 47,said bushing and sleeve being preferably formed of brass. The bushing 51is provided with a portion of reduced internal diameter 52 in which oneend of the coaxial lead 13 is snugly received, said lead being sealedtherein, as by silver soldering or brazing at 53.

Coaxially disposed within the sheath member 43 is a second thermoelement54 in the form of a cylindrical ingot of Semi-metallic material lto bedescribed in detail hereinafter. The thermoelement 54 is provided withcontact electrodes 55 and 56 for eifecting electrical connectiontherewith, the electrode 56 preferably having a fused or bondedelectrical connection with said thermoelement at 57, and having anelongated stem portion 58 extending coaxially within the sheath member43 and sleeve 47 as shown. The contact electrode 55 has a stem portion59 extending coaxially within the reduced diameter portion 44 of sheathmember 43 and is joined at its outer end to the outer end of saidreduced diameter portion, as by welding at 60. The contact electrode 55also has an enlarged head portion 61 seated against the annular shoulder45 and preferably formed with a concave conical surface 62. forreceiving the adjacent complementary convexV conical end surface 63formed on the thermoelement 54. The contact electrodes 55 and 56 arepreferably formed of material which will not react with the semimetallicthermoelement 54. Suitable materials from which the contact electrodesmay be made are iron, molybdenum, and alloys of iron and molybdenum,each of said materials preferably having only the normal minute amountsof residual impurity therein.

A bushing 64 of thermal and electrical insulating material has a slidingfit within the bore of the sleeve 47, and the end portion of the contactelectrode stem 58 extends through said bushing and is thereby maintainedcentered within the sleeve 47 and sheath 43. A metallic bushing 65 has asnug coaxial fit on the contact electrode stem 58 and is interposedbetween the head portion thereo'f and the insulating bushing 64 asshown. Extending coaxially within the sleeve 47 and interposed betweenthe insulating bushing 64 and bushing 51 is an elongated helicalcompression spring, preferably of heat resistant stainless steel, whichacts through the insulating bushing 64, metal sleeve 65 and contactelectrode 56 to place the thermoelement 54 under aXial compressionbetween the electrodes 55 and 56. The coaction of the conical surfaces62 and 63 on the electrode 55 and thermoelement 54, respectively,maintains the thermoelement 54 and the adjacent portions of the contactelectrode 56 and sleeve 65 centered within the sheath 43. The pressurecontact between the surfaces 62 and 63 affords a low resistanceelectrical connection between the thermoelement and contact electrode 55which functions as a 'hot thermojunction in the operation of thegenerator 4. The bondV 57 between the thermoelement 54 and the electrode56 and the fused connection 46 between the sheath 43 and sleeve 47 bothafiord 'cold thermojunctons in the operation of said generator.

It will be observed that there is a relatively small mass of metaladjacent and in good heat conducting con tact with the hot junctionaforementioned, i.e., the reduced diameter portion of the sheath 43 andthe contact electrode 55. On the other hand, there is a relativelymassive metallic assembly in good heat conducting contact with the coldthermojunction aforementioned, i.e., the contact electrode 56 with itselongated stem 58 and the metallic sleeve 65. It will also be observedthat the aforementioned relatively large metallic mass is maintained bythe insulating bushing 64 out of heat conducting contact with the sheath43 and sleeve 47, so that heat transfer between said mass and saidsheath and sleeve can take place only by radiation or through gaseousfill referred to hereinafter.

It will also be observed that the relatively short length of thethermoelement 54 affords a relatively small contact separation indicatedin Figure 2 by the letter A. This short contact separation permitssubstantial heat transfer through the thermoelement 54 in spite ofrelatively low thermal conductivity characteristics of the semimetallcmaterial from which it is made. In the preferred embodiment of theinvention, the aforementioned contact separation is preferably notgreater than the diameter of the cylindrical thermoelement 54. It willbe noted that the sheath member 43 has a relatively thin-walled portionindicated by the letter B in Figure 2 lthermally connecting the outerend portion of said sheath with a thcker-walled, 'and hence moreconductive, portion of the enclosing structure. In order to affordsubstantial heat transfer from the outer end portion of the sheathmember 43 to said more conductive enclosure portion, for example, to thesleeve 47, the length of the thinwalled sheath portion B is keptrelatively short. In the preferred embodiment of the invention thedimension B is preferably not greater than the outer diameter thereof.

The coaxial type thermoelectric generator lead 13 cornprises a tubularouter conductor 67 and a coaxial inner conductor 68 insulated therefromby an insulating sleeve 69 preferably made of non-hygroscopic material.The

;inner conductor .68, is connected in circuit with the stem 58 of thecontact electrode 56, as by a brazed or welded connection at 70, and theportion of the conductor 68 extending within the compression spring 66may be formed with a Serpentine shape as shown to provide extensibilityfor a purpose to be hereinafter described. The end of the lead 13 remotefrom the generator 4, in addition to carrying the connector nut 14, hasthe outer tubular lead conductor 67 radially outwardly flared, as at 71,for engagement by the end of said nut. The inner lead conductor 68 has alow resistance electrical connection with the terminal tip 15, and arelatively rigid insulating spacer 72 surrounds the inner lead conductorand is interposed between the tip 15 and the flange 71 to insulate saidtip from said flange while at the same time maintaining a predeterminedminimum spacing therebetween. The spacer 72 may be of glass melamine orother suitable material.

The insulation 69 between the inner and outer lead conductors 68 and 67terminates short of the flange 71, as at '73. Surrounding the inner leadconductor 68 within the outer lead conductor 67 and between theinsulating washer 72 and the end portion 73 of the insulation 69, is aninsulating Seal 74 of pliable material, for example, silicon rubber,which directly contacts both the inner and outer lead conductors. Priorto the connection of the lead 13 to the generator 4, and after theterminal tip 15 is fixed to the end of the inner conductor 68 followingplacement of the Seal 74 and insulating spacer 72 on. the conductor 68,an axial pull is exerted on the inner conductor to draw the spacer 72and tip 15 toward the flange 71 and thereby force the material of thepliable Seal 74 into the adjacent end of the outer conductor 67 and intosealing engagement with both the inner conductor 68 and said outerconductor. While the inner conductor is thus under tension, the outerconductor is crimped, as at 75, to frictionally clamp the innerconductor 68 through the insulation 69. This clamping engagementmaintains the portion of the inner conductor between the crimped portion75 and the tip 15 under continuous tension so that the Seal 74 iscontinuously pressed by the spacer 72 to maintain a continuous hermeticSeal between the conductors 67 and 68.

By virtue of the Seal 74 and the aforedescribed sealing connections at53, 50 and 46, an hermetically sealed sheath or envelope is providedwhich encloses the thermoelement 54 and contact electrodes 55 and 56,said sheath comprising the outer tubular conductor 67, tubular sheathmembers 51 and 47, and sheath member 43. This envelope or enclosuremeans preferably contains a reducing fill, for example methane or othersuitable hydrocarbon gas affording, when heated, hydrogen which tends toreduce oxidation within said enclosure means.

The thermoelement 54 is preferably made of semimetallic material havinghigh thermoelectric power, for example, an alloy further described inPatent No. 2,811,- 570 of Sebastian Karrer, said alloy comprising leadand at least one member of the group tellurium, Selenium and sulphur.For example, the thermoelement 54 of lead- Selenium-telluriumcomposition could include a Seleniumtellurium constituent in which theSelenium is but a trace. In this case such constituent should constitute35% to 38.05% by weight of the composition, the balance (61.95% to 65%by weight) being lead. At the other extreme, where theSelenium-tellurium constituent consists almost entirely of Selenium withbut a trace of tellurium, such constituent should comprise 25% to 27.55%by weight of the final composition, the remainder (from 72.45% to 75% byweight) being lead. Between these two cxtremes, the Selenium-telluriumconstituent varies linearly with the ratio of Selenium to tellurium(expressed in atomic percent) in the Selenium-tellurium constituent.

The thermoelement 54 may also be formed with an alloy of lead, Seleniumand sulphur. For example, a

thermoelement 54 of the lead-selenium-sulphur composition could consistof a selenium-sulphur constituent in .sired electrical and physicalproperties.

which the sulphur is but a trace. In this case, such constituent shouldconstitute from 25 to 27.55% by weight of the composition, the balance(from to 72.45% by weight) being lead. At the other extreme, where theselenium-Sulphur constituent consists almost entirely of sulphur withbut a trace of Selenium, such constituent should comprise from 12.8% to13.37% by weight of the final composition, the remainder (from 87.2% to86.63% by weight) being lead. Between these two extremes, theselenium-sulphur constituent varies linearly with the ratio of Seleniumto sulphur (expressed in atomic percent) in the selenium-sulphurconstituent.

With regard to the aforementioned compositions, it will be observed thatin each case there is an excess of lead over and above the amountthereof necessary for satisfying the stoichiometric proportions of thecompound formed in the second constituent or constituents, i.e., thetellurium, Selenium or sulphur. For example, the composition consistingsubstantially of lead and Selenium can contain up to 10.4% lead byweight of the total composition over and above the 72.41% by weight leadstoichiometrically necessary for combination with selenium.

The electrical characteristics of the aforementioned Semi-metallicalloys, desirable, for example, in thermoelectric generator elements,can be markedly and advantageously altered in a reproducible manner bythe addition thereto of Controlled amounts of matter other than theconstituents of the base composition. Such additions may also bedenominated beneficial irnpuritlesj, as distinguished from undesirableimpurties. For convenience, these additions are hereinafter designated'promoters, since they tend to enhance the electrical characteristicsdesired for the particular application of the base compositions.

The above-described base compositions exhibit negative thermoelectricpower and negative conductivity. By the addition of certain *promoterssuch negative properties may be enhanced, while the polarity of theelectrical properties of the base composition may be reversed by theaddition of certain other promoters to provide a Semi-metalliccomposition having positive electrical characteristics. Patent No.2,8l1,57l, of Robert W. Fritts and Sebastian Karrer, gives a completedescription of the beneficial impurities, including both departures fromperfect Stoichiometry and promoters, which have been found to beeffective for mprovement of electrical properties of the Semi-metallicthermoelectric generator elements when added to the aforementioned basecompositions in minor amounts, for example, up to a maximum of 69% byweight of beneficial impurity including 3.9% excess lead and 3.0%promoter.

The proportions and ranges of the various constituents aforementioned,and particularly the minimum limits of lead constituent in the basecompositions, must be regarded as critical if the composition is to havethe de- If the lead content is significantly less than the mnimum amountindicated for any particular Selenium-tellurium or Seleniumsulphurproportion, the polarity of the Seebeck changes, and the desiredelectrical and mechanical properties Will not be reproducible. On theother hand, if the lead content of any composition appreciably exceedsthe aforementioned maximum limits, the resulting composition is toometallic in nature to afford satisfactory energy Conversionefficiencies. Not only are the proportions and ranges of theaforedescribed compositions to be considered critical but so also is thepurity. .More specifically, the limit of tolerable metallic impurity innon-promoted final compositions has been found to be on the order of0.01%, and the composition must be substantially Oxygen free, if themechanical and electrical properties desired are to be obtained and areto be reproducible. In the case of promoted compositions, however, thelimit of tolerable impurity is 0.001%.

The fuel control system shown in Figure 1 is placed in operation bymanually resetting the safety shut-off valve 9 to permit fuel flow tothe pilot burner 6. Ignition of the `fuel at the pilot burner 6 causesimpingement of the pilot burner flame upon the reduced diameter portion44 'of the sheath member 43 of the generator 4, whereupon saidgenerator, upon heating of the hot junction at the surfaces 62 63,generates electrical energy sufficient to energize the windings of theelectromagn'ets 17 and 33 of valves 9 and 7 respectively. Energizationof the electromagnet 17 causes the latter to retain the armature 18 inattracted position and the valve 19 in open position after release ofthe reset means 23, permitting continuous fuel flow to the pilot burner6. The pressure regulator 11 insures that the supply of fuel to thepilot burner 6 is maintained at relatively Constant pressure, so thatnniform heating of the generator 4 results. After energizaton of theelectromagnet 33, the valve 7 may be reset, the armature 34 beingretained in attracted position and the valve 36 in open position by saidelectromagnet following release of the reset means 39, to affordcontinuous fuel flow to the burner 5.

During application of heat to the outer end portion of the sheath 43substantial quantities of heat flow through the thermoelement 54 intothe relatively massive metallic assembly afforded by the contactelectrode S6, its stem 58 and the metallic sleeve 65, and as a result,said assembly acquires a relatively large heat content. In contrast tothis, the relatively small mass of the contact electr'ode 55 and thereduced diameter portion 44 of the sheath 43 acquires only a relativelysmall heat content even though its Operating temperature issubstantially higher than that of said assembly. While the substantialheat flow through the thermoelement 54 results in a temperaturedfferential between the hot and "cold thermojunctons thereof which isless than that found in the operation of conventional thermoelectricgenerators, and those having longer thermoelements, the highthermoelectric power of the Semi-metallic thermoelement materialnev'ertheless aflords an output which is greater than that afforded byconventional metallic thermoc'ouples, for example, those utilizingstainless steel and copel thermoelements. I

During the operation of the generator 4, prolonged exposure to heat maytend to cause a certain amount of sublimation and resultant shorteningof the Semi-metallic thermoelement 54. Deleterious results from any suchaction are prevented, however, by the elon'gated lielical spring 66which, in exerting a compressive stress o'f approximately lbs. upon thethermoelement 54, maintains the low resistance contact between thesurfaces 62 63 and aifords take-up of any length reduction in thethermoelement 54. The elongated nature of the helical spring 66 providesfor a relatively large amount of such take-up and thereby insuresprolonged life expectancy for the generator 4. 4

Upon extinguishment of the flame at the pilot burner 6, the outer endportion of the generator 4 cools rapidly to quickly reduce thedifferential in temperature between the *hot thermojunction 62-63 andthe 'cold thermojunction 57 with a simultaneous rapid drop-off in theoutput of the generator to the drop-out value efec'ting closure of thevalves 7 and 9 and termination of all fuel flow to the main and pilotburners. As iilustrative of the rapid action of systems embodyingimproved thermoelectric generator 4, safety shut oif of the fuel in onesuch system was effected Within four seconds of extinguishment of thepilot burner flame. This compares with a time interval of forty secondsrequiredin systems having as their source of power standard metallicther'rnocouples, for example, those having stainless steel and copelthermoelements.

The 'aforernentioned rapid response of the improved thermoele'ctricgenerator 4 to extinguishme'nt of the pilot burner flarne isattributable to structural features effectihg a rapid `reduction in thereduced Operating temperature ditfer'ential between th'e '*hot and'fcold thermojunc't-ions 'of the generator. On out'age of the pilotburner fiarne', heat is yra'di'ated from the hot relatively small massouter fe'nd of rthe generator 2%, the maximum amount of such radiative'cool'ing' being aforded by the' m'ounting of said geneator in arelatively open environment to provide lthe maximum vsolid angle throughwhich heat can radiate from the aforementioned generator end portion.Heat is also conducted away from the relatively small mass outer endportion of the generator through the sheath, member S to the coolersleeve 47, as well as from the contact electrode 55 through thethermoelement 54- and into the cooler relatively massive metallicase'mbl'y atford'ed by the contact electrode 56, and the sl'eeve 65. Thelat-iefi'tiohed massive metallic ssernbly is prevented from coolingrapidly during the rapid lcooling of the outer end portion of thegenerator, not only because of the large heat content thereof madepossible by its large inass', but also because of the fact that saidassembly is held out of direct contact With the cooler portions of thesheath member 43 land sleeve 47 by the insulatihg bushing 64 andtherefore heat cannot be condiicted therefrom to said members. Thus, thedifferential between the hot and *cold thermojunctiohs of thethermoelement 54 is' rapidly reduced from its `Operating value to effecta c'orresponding drop-off in the electrical output of the generator 4 asa result of the aforementioned rapid cooling of the hot junc'tion andsimultaneous maintenance of the temperature of the *cold thermojunctionof the thermoelement 54 near its operating value.

The improved thermoelectric generator 4, because of the simplicity ofits construction can be fabricated by mass production techniques fromrelativelyinexpenslve components; It is sturdy in construction and, ifdesred, can be made interchangeable with presently installedconventional metallic thermocouples to impart rapid dropoutcharacteristics to the systems into which they are introduced.

What is claimed as the invention is:

1. A thermoelectric generator comprising a cup-shaped metallic outerthermoelement, a tubular member of metal different from that of saidouter thermoelement coaxially joined at one end to the open end of saidcup-shaped thermoelement to form therewith a sheath and aifording atsaid juncture an outer cold thermo'junction, a semimetallic innerthermoelement coaxially disposed within said cup-'shaped thermoelement,means forrning a hot thermo'junctio'n between the closed end of saidcupshaped thermoelement and the adjacent end of said innerthermoelement, and metallic heat storing and thermojunction meansformin'g an inner cold thermojunction With the opposite end of saidsernimetallic inner thermoelement and extending axially in spacedrelation Within said outer thermoelement and tubular member, said heatstoriug and therm'ojunction means having a cross-sectional area of asize to occupy the major pfortion of the crosssectional area of thehollow interio'r of the portion of the sheath in which it is disposedand having a length substantially greater than the axial distancebetween said hot th'ermojunctio'n and said outer cold' thei'mojunction.

2. A thermde'lectric generator comprising a cup-shaped metallic outerthermoelement, a tubular member of metal different from that of saidouter thermoelement coaxially joined at one end to the open end of saidcup-shap'ed thermoelement to form therewith a sheath and affordin'g atsaid juncture an outer cold thermojunction, a cylindrical semimetallicinner thermoelement coaxially disposed within said c'up-shape'dthermoelement and' having an axial length not substantially greater thanits diameter, means forming a hot thermojunction between the closed endof said c'up-shaped thermoelement and the adjacent end of said innerthermoelement, and metallic heat storing and thermojuncton means formingan inner cold thermojunction with the opposite end of said 'semimetallicinner thermoelement and extending axially in spaced relation Within saidouter thermoelement and tubular member, said heat storing andthermojunction means having a cross-sectional area of a size to occupythe major portion of the cross-sectional area of the hollow interior ofthe portion of the sheath in which it is disposed and having a lengthsubstantially greater than the axial dlstance between said hotthermojunction and said outer cold thermojunction.

3. A thermoelectric generator comprising a cup-shaped metallic outerthermoelement, a tubular member of metal different from that of saidouter thermoelement ooaxially joined at one end to the open end of saidcup-shaped thermoelement to form therewith a sheath and aifording atsaid juncture an outer cold therrnojunction, a semimetallic innerthermoelement coaxially disposed within said cup-shaped thermoelement,means forrning a hot thermojunction between the closed end of saidcup-shaped thermoelement and the adjacent end of said innerthermoelement, the axial distance between said hot thermojunction andsaid outer cold thermojunction being not substantially greater than theouter diameter of said outer thermoelement, and metallic heat storingand thermojunction means forming an inner cold thermojunction with theopposite end of said semimetallic inner thermoelement and extendingaxi'ally in spaced relation within said outer thermoelement and tubularmember, said heat storing and thermojunction means having across-sectional area of a size to occupy the major portion of thecross-sectional area of the hollow interior of the portion of the sheathin which it is disposed and having a length substantially greater thanthe axial distance between said hot thermojunction and said outer coldthermojunction.

4. A thermoelectric generator comprising a relatively thin-walledcup-shaped metallic outer thermoelernent, a thicker-walled tubularmember of metal different from that of said outer thermoelement andhaving greater thermal conductivity than said thermoelement coaxiallyjoined at one end to the open -end of said cup-shaped thermoelement toform therewith a sheath and affording at said juncture an outer coldtherm'ojunction, a semimetallic inner thermoelement coaxially disposedwithin said cup-shaped thermoelement, means forming a hot thermojunctionbetween the closed end of said cup'shaped thermoelement and the adjacentend of said inner thermoelement, and metallic heat storing andthermojunction means forming an inner cold thermojunction with theopposite end of said semimetallic inner thermoelement and extendingaxially in spaced relation within said outer thermoelement and tubularmember, said heat storing and thermojunction means being insulatablysupported within said sheath and having a cross-sectional area of a sizeto occupy the major portion of the crosssectional area of the hollowinterior of the portion of the sheath in which it is disposed and havinga length substantially greater than the axial distance between said hotthermojunction and said outer cold thermojunction.

5. A thermoelectric generator comprising a relatively thin-walledcup-shaped metallic outer thermoelement, a thicker-walled tubular memberof metal different from that of said outer thermoelernent and havinggreater thermal conductivity than said thermoelement coaxially joined atone end to the open end of said cup-shaped thermoelement to formtherewith a sheath and affordlng at said juncture an outer coldthermojuncton, a cylindrical semimetallic inner thermoelement ooaxiallydisposed within said cup-shaped thermoelement and having an axial lengthnot substantially greater than its dameter, means forming a hotthermojunction between the closed end of said cup-shaped thermo'elementand the adjacent end of said inner thermoelement, the axial distancebetween said hot thermojunction and said outer cold thermojunction beingnot substantially greater than the outer diameter of said outerthermoelement, and metallic heat storing and thermojunction meansforming an inner cold thermojunction with the opposite end of saidsemimetallic inner therrnoelement and extending axially in spacedrelation within said outer thermoelement and tubular member, said heatstoring and thermojunction means being insulatably supported within saidsheath and having a cross-sectional area of a size to occupy the majorportion of the cross-sectional area of the hollow interior of theportion of the sheath in which it is disposed and having a lengthsubstantially greater than the axial distance between said hotthermojunction and said outer cold thermojunction.

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